There are believed to be various gas sensors, as used for analysis of the exhaust gases of internal combustion engines and, among other things, also for the analysis or detecting the presence of nitrogen oxides (NOx). A survey may be found in F. Ménil et al., Sensors and Actuators B, 67 (2000), pages 1 to 23. A gas sensor for detecting nitrogen oxides is also discussed in European Patent Application No. 0 678 740, and apparently works on the principle of a limiting current sensor and has a double chamber arrangement, in which chambers are separated from each other by a diffusion barrier.
In such a limiting current sensor, the limiting currents, which appear at a typical concentration of nitrogen oxides in an exhaust gas of an internal combustion engine of 500 ppm, lie around ca. 7 μA, so that they can be processed further, and used as control or regulating quantities only with difficulty by customary motor vehicle electronic systems.
The functioning of limiting current sensors is based on applying a constant voltage between two electrodes separated from each other by a solid electrolyte, or, in the case of a three-electrode configuration, setting it by using a potentiostat. This voltage leads to an electrical current between two electrodes which is based on electrochemical conversion of gas components, or rather exhaust gas components.